Good inputs here. "Air filter upgrades" are all pretty much scams.
But a perfect money maker at that.... just look at all the companies vying for your $$$.
Here is some real data on K&N: http://www.billswebspace.com/AirFilterTest.htm
Unfortunately for the "My brother swears by it crowd" it likely requires WAY too much critical thinking....
Keep it as simple as possible...but no simpler.